The present invention relates to the field of anchor chains, and more particularly to anchor chains for applications at sea.
In the present context, the term “anchor chain” is used to mean a succession of links connected to one another for transmitting mechanical tension, and serving in particular to connect a floating body to an anchor point in order to restrict the movement of said floating body. The links of such an anchor chain may in particular be made of metal. They may be stud links, in order to avoid them being flattened, or they may be links without studs.
Although anchor chains are designed to transmit mechanical forces mainly in tension, in practice, chains anchoring a floating body and subjected to high levels of tension can also be subjected, at their ends, to large bending stresses, in particular in the proximity of their connection with the floating body. The cause of this bending is the combination of pivoting movements of the floating body and of geometrical imperfections in the areas of contact between adjacent links.
Specifically, the imperfections in combination with high levels of tension prevent hinging between adjacent links. Movements of the floating body thus lead to lateral forces on the anchor chain and to bending moments that are transmitted between adjacent links. In the long term, such recurrent bending can in particular give rise to breaking by fatigue, as described in particular in the article “Failure of chains by bending in deepwater mooring systems”, presented by P. Jean, K. Goosens, and D. L'Hostis at the 2005 Offshore Technology Conference in Houston, Tex., USA.
In the prior art, in order to avoid such fatigue, and thus lengthen the lifetime of anchor chains, attempts have been made to limit the bending movements to which the links of the chain are subjected by improving the hinges at the ends of chains or by locally modifying the links.
Thus, by way of example, in international patent applications WO 2010/112603 and WO 98/40306, proposals are made to interpose universal joint connectors and long lever arms between the end of the anchor chain and the floating body. Nevertheless, such connectors present the drawback of being heavy and voluminous, which can make them difficult to integrate. Also, the cost of universal joints, in particular if they are to withstand a hostile environment such as seawater, is very high.
Proposals have also be made in particular in international patent application WO 2008/0951106 to interpose, not hinged joints, but rather segments of cable between the end of the anchor chain and the floating body. Nevertheless, that normally presents the drawback of requiring the length of the cable segment to be determined accurately before anchoring the floating body, and thus also of requiring the position of the anchor point to be determined accurately.
Devices and methods have also be proposed to replace segments of chain in operation before they reach a fatigue threshold. By way of example, that can be achieved by passing such segments through a deflector sheave or a specific fairlead and by changing the links bearing against the connection by means of a winch, e.g. as shown in French patent application FR 2 601 322. Nevertheless, that normally requires heavy and expensive devices to be installed on the floating body, and also storage for replacement segments of chain. Furthermore, the links of those replacement segments of chain can also be damaged by passing through such devices.